exo-open mailto: without recipient address fails to start Mail Reader

Something like:$ exo-open mailto:you@me.com?subject=fooloads my Mail Reader properly, but an absent or ill formed recipient as in :$ exo-open mailto:?subject=foofails with a dialog stating: Unable to detect the URI-scheme...I had read somewhere that an empty recipient field was legal. Is this a bug?

Okay, looks like there is a known bug for this: https://bugzilla.xfce.org/show_bug.cgi?id=10098. Arch has already patched exo with this fix (which explains why it works for me), but the official 0.10.2 version does not include this patch (which explains why it doesn't work for you).